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Air-based systems are generally relatively low-tech, require shallow interventions in the ground and are suitable for most sites. They can often be implemented relatively cheaply with little mechanical equipment required. However, they tend to be less well-known than water-based systems, and their behaviour is not as well understood, in particular the effect of the system itself on the temperatures of buried air paths.

The heat transfer achieved depends on the surface area available, the velocity (which to a large extent determines the heat transfer coefficient), the time spent in contact with the ground and the external temperature. These are often competing with each other, but the critical issue in any system is to ensure that the airflow in the ground contact zone is turbulent. For a typical floorplenum 300mm deep, this occurs at a velocity of as little as 0.2 m/s, reducing proportionately in larger ducts and increasing in smaller pipes. Once in the turbulent zone, the heat transfer coefficient will continue to increase as the air moves faster. However, it can still obey the law of diminishing returns, with high velocities increasing pressure losses, resulting in noisier fans and more electrical energy usage.

Analysis of the airside heat transfer can be performed relatively simply, but assessment of how the temperature of the ground varies with depth and time is more difficult, and requires detailed thermal modelling in four dimensions. As a general rule, the deeper the ducts the better, up to a depth of around 2-3m.

Earth tubes are pipes buried in the ground. This is a cheap technology that can be easily utilised beneath the footprint of a building or in its surroundings. Burial depths of 2m are preferred, but 1m of cover can provide a good degree of stability.

A more powerful technique that can harness the entire footprint of a building is the use of labyrinths and undercrofts. These can be located under the building, or even extend beyond it, and can be deep enough to allow access for maintenance. The most well-known labyrinth scheme is probably the one at the Earth Centre in Doncaster, where the walls even have an irregular shape to increase turbulence and the surface area available for heat transfer.

Earth tubes can be dealt with in a similar way to drains, with access necessary at both ends, and preferably a pit at one end. CCTV can be used to check for defects, and spray methods can be used to clean the ducts. To ensure that condensation is managed efficiently, the ducts need to be laid with a slight fall.

Labyrinths are best dealt with by making them tall enough to be accessible. If this is not possible, regular access points into shallower voids should be provided.

It is crucial to ensure that dirt and vermin ingress is avoided by sealing any inadvertent entries to ducts.

If a building is mechanically ventilated, the additional cost of an undercroft or earth tubes can be relatively minor. They do not generally involve significant technology.

One issue with undercroft schemes is that while they recover heat during the winter, and coolth in the summer, there is a period in between when the air is likely to be cooled down by passing across the ground and will then need to be reheated before entering the building. This can be addressed by allowing the intake air to be taken either from the ground source or directly from outside, enabling the optimum balance of conditions, and reducing energy use further.

The thermal storage effect of large underground ducts can be increased by placing gabions within them, which provide an increased surface area and create a more turbulent flow.

It is important to make sure air flow is turbulent, but not so fast as to generate large pressure drops.

It is important to ensure that clients and occupants are aware of the nature of the system so that they can be ‘on board’ and supportive as it settles down, particularly in the first few months of use.

This article was created by --Buro Happold, 17 March 2013, based on a 2008 article in 'Patterns'.